1. Field of Invention
This invention relates generally to methods for reducing plate-out during the process used to produce stretch blow molded containers from polyester preforms and particularly to methods for reducing plate-out by crystallizing low molecular weight polyester molecules in the preform exterior surface.
2. Description of Related Art
Heat-set stretch blow molded containers are made using methods that yield containers with a high degree of thermal stability, i.e., minimal shrinkage after hot-filling. These containers can be hot-filled, pasteurized, washed at high temperatures, or used for any other applications where a high degree of thermal stability is required. These containers must be useful in processes that would distort normal polyester containers, particularly poly(ethyleneterephthalate) (xe2x80x9cPETxe2x80x9d) carbonated soft drink (CSD) bottles.
During the process of preparing heat-set stretch blow molded containers, the container preform is heated to higher temperatures than are normal for a CSD bottles. Normal preform skin temperatures for CSD bottles at the blow station are 20xc2x0 to 25xc2x0 C. above the glass transition temperature, i.e., about 100 to about 105xc2x0 C. In the heat-set process, the preform skin temperature at the blow station can be as high as 30-35xc2x0 C. above the glass transition temperature, i.e., about 110 to about 115xc2x0 C. The blow mold temperature is also much higher. In a CSD process, the mold is usually maintained at about 10xc2x0 C. In contrast, in a heat-set process the mold is elevated to about 110xc2x0 C. to about 140xc2x0 C. The mold surface contact time is also greatly increased to increase container crystallinity.
At these higher preform and mold temperatures, low molecular weight molecules on or in the polyester preform outer surface (i.e., mainly cyclic trimer and other linear low molecular weight species such as dimer, trimer, tetramer, etc.) become very mobile and tacky. These low molecular weight molecules leave the surface of the preform and adhere to the surface of the mold. Over time, the amount of these low molecular weight molecules adhering to the mold surface increases. The temperature of the mold surface is sufficient to induce thermal crystallization of these species and also ring-opening polymerization. As these deposits crystallize, they become very hard. Also, they build up sufficiently on the surface until they impart imperfections into the bottle surface as well as adhering to the bottle surface. The imperfections and crystallized particles refract light and cause undesired haze in the bottle surface. At some point during production, the stretch blow heat-setting process must be stopped to clean this plate-out deposit from the mold surface. For some current processes, cleaning the molds is conducted as often as once a day.
While some art exists indicating solvent or thermal crystallization of polyesters to improve the thermal stability of polyester bottles, no art indicates that crystallizing the surface will decrease mold plate-out. JP 3207748 and JP 216081 disclose adding a small amount of polyamide nucleator to aid crystallization of the entire thickness of the bottle during the heat-set process to improve thermal stability. However, there is no mention of any improvement in reducing mold plate-out or any reason to preferably crystallize the skin of the preform only. U.S. Pat. No. 5,090,180 discloses thermally crystallizing the entire thickness of the base during the stretch blow process to improve thermal and mechanical stability of the bottle, however, nothing is said about decreasing mold plate-out. JP 62030019 discloses thermally crystallizing the entire bottle before the second stretch blow step of a two step stretch blow process. The resulting bottle is disclosed to have reduced internal residual strain and a low degree of haze, however, there is no mention of any improvement in mold plate-out. JP 58119829 discloses passing the preform through a flame treatment to melt the surface, which should cause some thermal crystallization, and reduce surface defects without imparting haze. However, there is no mention of a reduction in mold plate-out.
JP 56150516 and JP 53110669 disclose solvent crystallizing the neck and mouth of the bottle, after the stretch blow process, to improve solvent-crack resistance in the bottle without increasing the haze level in those regions. However, there is no mention of reducing mold plate-out. DE 19934320-A1 discloses that blowing the preform with superheated air and decreasing mold temperature significantly produces a thermally stable bottle with reduced plate-out for heat-set applications. Crystallizing the preform outer surface is not disclosed. WO 01/19594 discloses inducing crystallinity in a plastic container by heating an interior surface of the plastic container. None of these references disclose methods for reducing or eliminating plate-out. There is, therefore, a need for methods for eliminating plate-out.
It is, therefore, an object of the present invention to provide a method for reducing or eliminating plate-out during the process used to produce stretch blow molded containers from polyester preforms.
It is another object of the present invention to provide a preform that will reduce or eliminate plate-out during the process used to produce stretch blow molded containers from polyester preforms.
It is another object of the present invention to provide a method for making blow molded containers from polyester performs having from about 0.01% to about 2% low molecular weight polyester molecules in the preform.
These and other objects are achieved using a method that crystallizes low molecular weight polyester molecules in or on the preform exterior surface (with the exception of the support ring and the finish) before stretch blow molding the preform into a container. The molecules are crystallized using a crystallization process selected from (1) treating the outer surface of the preform with a solvent that is capable of crystallizing low molecular weight polyester molecules in polyester or (2) heating the outer surface of the preform to a temperature and for a time suitable for crystallizing low molecular weight polyester molecules in polyester. The crystallized molecules do not migrate out of the preform and form plate-out deposits on the mold. It has been surprisingly found that the method of the present invention reduces, and in some situations eliminates, the need to stop the heat-set stretch blow mold process because of mold plate-out.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art.